The present invention relates to a type of vertical overtravel stop for a MEMS microphone which does not incorporate the substrate and requires no dedicated insulation layer or special electrical measures to avoid electric shorts during an overtravel event.
Capacitive MEMS microphones are mechanically extremely sensitive devices. They need to operate in a very high dynamic range of 60-80 db ( 1/1000- 1/10000). To create a membrane which is sensitive enough to detect the lowest pressures (˜1 mPa), it must be very compliant to pressure changes. At the same time, the membrane must withstand pressures in the range of several 10s of Pascals without being destroyed. This is typically achieved by clamping the membrane between overtravel stops (OTSs) in both directions. While an OTS towards the backplate (i.e., when the membrane is moving towards the backplate) is relatively easy to realize, the opposite direction (i.e., OTS towards the substrate, when the membrane is moving towards the substrate) either requires another dedicated layer or (typically) uses the substrate as the OTS.
FIG. 1 illustrates a typical capacitive MEMS microphone 100. The microphone 100 includes a backplate 105, a membrane 110, and a substrate 115. The membrane 110 is coupled to the backplate 105 at point 120 (the membrane 110 is insulated from the backplate 105 as they are at different electrical potentials). Sound waves passing through the backplate 105 cause the membrane 110 to vibrate up (in the direction of arrow 125) and down (in the direction of arrow 130). To prevent the membrane 110 from traveling too far toward the backplate 105, shorting the membrane 110 to the backplate 105, overtravel stops (OTSs) 135 are provided at both ends of the membrane 110. Each OTS 135 is sometimes referred to as “an OTS toward the backplate.” In addition, the substrate 115 itself provides a second OTS (“an OTS toward the substrate”).
During microphone operation, a high bias voltage (e.g., 1 to 40 V) is typically applied between the membrane 110 and the backplate 105. To avoid a short and potential destruction of the electronics, or the MEMS structure itself, series resistors or insulating layers on top of the OTS bumps are required. The use of series resistors requires careful design of the electronics, and the use of insulating layers increases the complexity/cost of the device significantly and may even be impossible due to process constraints. In addition, an insulating layer on top of the bumps is not an ideal solution as long as the membrane and the OTS bump are on different electrical potentials. In this case, electrostatic forces can decrease the pull-in voltage and/or provide sufficient force to keep the membrane 110 stuck to the backplate 105 after contact due to overload. Additional circuitry may be required to detect this and switch off the bias voltage to allow the membrane 110 to release from the backplate 105.
Creating the OTS towards the substrate is especially difficult. Due to processing tolerances during the backside processing, which typically incorporates a high rate trench, accommodations must be made to compensate for possible misalignment. FIG. 2A shows how the trench can vary from the frontside 200 to the backside 205. To accommodate for the typical misalignment 210 between the frontside 200 and the backside 205, the membrane 110 and the substrate 115 have a large, e.g., several microns, overlap. Additionally, the variation of the backside trench leads to a large variation at the deep end of the trench, and adds to the overall tolerances (several tens of microns). The accuracy of the backside trench can be improved at the cost of processing time. Longer processing increases the device's cost.
FIG. 2B shows another solution to this technical problem. A two-step backside trench 215 is used. This results in sufficient accuracy, but doubles the cost of this processing step.
Overlapping of the membrane 110 and the substrate 115 results in a significant and varying parasitic capacitance which directly influences the final sensitivity of the sensor element. Accordingly, it is important to keep the overlap of the membrane 110 and the substrate 115 to a minimum.